So-called “dry fermentation” allows pourable biomasses from agriculture, from biological waste and from communal cultivated areas to be converted to methane without having to convert the materials to a liquid substrate which can be pumped. Biomasses with a dry substance component of up to 50% can be fermented. This dry fermentation method is described, for example, in EP 0 934 998.
In the case of “dry” fermentation, the material to be fermented is not stirred into a liquid phase as is the case, for example, with liquid fermentation of bio waste. Instead of this, the fermentation substrate which has been introduced into the fermenter is kept moist all the time by taking the percolate at the bottom of the fermenter away and spraying it over the biomass again. This results in optimum living conditions for the bacteria. During the recirculation of the percolate, the temperature can also be regulated, and it is possible to add additives for process optimisation.
WO 02/06439 discloses a bioreactor or a fermenter in the form of a prefabricated garage which is operated using the principle of dry fermentation in the so-called batch process. In this case, after seeding with already fermented material, the fermenter is filled with the fermentation substrate by means of tractor shovels. The fermentation container is constructed in the form of a garage and is closed by a gastight door. The biomass is fermented with air being excluded, with no further thorough mixing during the process, and with no additional material being supplied. The percolate which seeps out of the material being fermented is drawn off via a drainage groove, is temporarily stored in a tank, and is sprayed over the fermentation substrate again, in order to moisturise it. The fermentation process takes place in the mesophilic temperature range between 34 and 37° C., with the temperature being created by means of bottom heating and wall heating.
The resultant biogas can be used to obtain electricity and heat in a cogeneration system that generates heat and electric power at the same time. In order to ensure that sufficient biogas is always available for the cogeneration system, a plurality of fermentation containers are operated with offset timings in the dry fermentation installation. At the end of the dwell time, the fermenter area is completely emptied and is then refilled. The fermented substrate is supplied to a post-composting process, resulting in an organic fertiliser that is comparable to conventional compost.
The batch operation means the individual fermenters must be shut down from time to time, that is to say the biogas production must be stopped, the fermented biomass must be removed from the respective fermenter, and the fermenter must be filled with fresh biomass, with biogas production being resumed. During this process, it is necessary for safety reasons to prevent an explosive biogas/air mixture from being created while the individual fermenters are being loaded and unloaded.
For this purpose, it is known from EP 1301583 B for a fermenter that is in use to be flooded with off-gas containing carbon dioxide from the cogeneration system that is being operated with biogas, in the event of an explosion risk, that is to say if air has entered the fermenter.